title: Nonmedical vaccine exemptions and pertussis in California, 2010.
abstract: BACKGROUND
  In 2010, 9120 cases of pertussis were reported in California, more than any year since 1947. Although this resurgence has been widely attributed to waning immunity of the acellular vaccine, the role of vaccine refusal has not been explored in the published literature. Many factors likely contributed to the outbreak, including the cyclical nature of pertussis, improved diagnosis, and waning immunity; however, it is important to understand if clustering of unvaccinated individuals also played a role.
  
  
  METHODS
  We analyzed nonmedical exemptions (NMEs) for children entering kindergarten from 2005 through 2010 and pertussis cases with onset in 2010 in California to determine if NMEs increased in that period, if children obtaining NMEs clustered spatially, if pertussis cases clustered spatially and temporally, and if there was statistically significant overlap between clusters of NMEs and cases.
  
  
  RESULTS
  Kulldorff's scan statistics identified 39 statistically significant clusters of high NME rates and 2 statistically significant clusters of pertussis cases in this time period. Census tracts within an exemptions cluster were 2.5 times more likely to be in a pertussis cluster (odds ratio = 2.47, 95% confidence interval: 2.22-2.75). More cases occurred within as compared with outside exemptions clusters (incident rate ratios = 1.20, 95% confidence interval: 1.10-1.30). The association remained significant after adjustment for demographic factors. NMEs clustered spatially and were associated with clusters of pertussis cases.
  
  
  CONCLUSIONS
  Our data suggest clustering of NMEs may have been 1 of several factors in the 2010 California pertussis resurgence.
authors.affiliation: MBBS, 1518 Clifton Rd, NE, Room 7017, Atlanta, GA 30322. somer@emory.edu.
authors.forename: Jessica E
authors.initials: JE
authors.lastname: Atwell
authors.forename: Josh
authors.initials: J
authors.lastname: Van Otterloo
authors.forename: Jennifer
authors.initials: J
authors.lastname: Zipprich
authors.forename: Kathleen
authors.initials: K
authors.lastname: Winter
authors.forename: Kathleen
authors.initials: K
authors.lastname: Harriman
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Salmon
authors.forename: Neal A
authors.initials: NA
authors.lastname: Halsey
authors.forename: Saad B
authors.initials: SB
authors.lastname: Omer
mesh_terms: D002140:California / Q000453:epidemiology; D002675:Child, Preschool; D016000:Cluster Analysis; D003469:Culture; D005260:Female; D006801:Humans; D008297:Male; D010567:Pertussis Vaccine / Q000627:therapeutic use*; D014611:Vaccination / Q000941:ethics; D014917:Whooping Cough / Q000175:diagnosis / Q000453:epidemiology* / Q000517:prevention & control*
chemical_list: D010567:Pertussis Vaccine
keywords: California; cluster analysis; immunization; nonmedical vaccine exemptions; pertussis; vaccine refusal